A heat-siphon type exhaust heat recovery device has been known as disclosed in, for example, JP-A-7-120178. The exhaust heat recovery device is constructed of a heat siphon including an evaporator and a condenser which are connected in a round shape. The evaporator is disposed in an exhaust pipe of an engine, while the condenser is disposed in an engine coolant pipe, so as to recover the heat of exhaust gas of the engine into the engine coolant.
Generally, a catalyst (catalytic converter) is provided for cleaning the exhaust gas at a midway point in the exhaust pipe. However, JP-A-7-120178 fails to disclose an effective arrangement of the evaporator or a suitable shape or the like of the exhaust pipe, taking into consideration such the catalyst.
For example, the catalyst functions only when the temperature of exhaust gas is equal to or higher than a predetermined temperature. When the evaporator is disposed on the upstream side of the exhaust gas flow from the catalyst, the exhaust gas is cooled by the evaporator, and thus the catalyst cannot sufficiently perform its function.
Since the evaporator is disposed in the exhaust pipe, the exhaust pipe has its size enlarged at an inlet and shrunk at an outlet with respect to the evaporator, thereby causing a loss in pressure of the exhaust gas through circulation. Further, an exhaust heat recovery device is desired which can effectively mount the catalyst and the evaporator on the vehicle or the like.
A loop heat-pipe type heat exchanger is proposed as a heat exchanger using the principle of the heat pipe, as disclosed in, for example, JP-A-4-45393. The heat exchanger includes a sealed circulation path for forming a closed loop, and a heat-transfer fluid sealed into the circulation path and being capable of being evaporated and condensed. The heat exchanger also includes an evaporator disposed in the circulation path for evaporating a working fluid using heat taken thereinto from the outside, and a condenser disposed in a higher position than that of the evaporator in the circulation path for exchanging heat between the heat-transfer fluid evaporated by the evaporator and a fluid to receive the heat transferred from the outside.
In order to provide an exhaust heat recovery device with a simple and compact structure having an advantage in mounting property on the vehicle, it is desirable that the evaporator and the condenser are integrally constructed. Taking one example, as shown in FIG. 8, an evaporator J1 and a condenser J2 are disposed adjacent to each other in the horizontal direction, and headers (connection portions) J5 are provided for communicating between both respective ends of the evaporator J1 and the condenser J2 in the vertical direction of heat pipes J3.
In the above-described exhaust heat recovery device, a working fluid evaporated by the evaporator J1 flows into the condenser J2 through the upper side header J5. The working fluid is condensed at the condenser J2 to become liquid, which flows into the evaporator J1 through the lower side header J5. A difference in water height (a difference in height of a water head h) of the working fluid (liquid) is caused between the evaporator J1 and the condenser J2 by a balance between the evaporation of the working fluid at the above evaporator J1 and the condensation of the working fluid at the condenser J2. This difference in water head height “h” causes the working fluid to be returned from the condenser J2 to the evaporator J1, so as to allow the circulation of the working fluid. Thus, in order to return a sufficient amount of working fluid from the condenser J2 to the evaporator J1, it is necessary to ensure the difference in water head height “h”.
The above-described exhaust heat recovery device exchanges heat between the working fluid evaporated by the evaporator J1 and the engine coolant at the upstream side in the condenser J2. Thus, in order to ensure a heat exchanging property of the condenser J2, a heat radiation property of the upper part of the heat pipe J3, positioned at the condenser J2 (that is, on a side into which the working fluid evaporated by the evaporator J1 flows), needs to be assured.